Field Project in Western Montana During the summers of 2002 and 2004, nine people contracted tick-borne relapsing fever on Wild Horse Island in southwest Flathead Lake, Lake County, Montana (Schwan et al, 2003, Emerg. Infect. Dis;Schwan et al., 2007;Emerg. Infect. Dis). Investigations by our group associated with these outbreaks resulted in the first isolations of Borrelia hermsii and the first collection of its tick vector, Ornithodoros hermsi, in Montana. The prevalence and distribution of B. hermsii in western Montana are unknown. The purpose of this project is to investigate the presence of B. hermsii in small mammal populations at two locations on Flathead Lake;Wild Horse Island, where the patients were infected;and the Flathead Biological Research Station, owned and run by the University of Montana, at Yellow Bay on the eastern shore of the lake. Information gained regarding the presence of B. hermsii in small mammals and ticks will help elucidate the ecological factors maintaining the spirochete, which could help develop future control strategies in specific locations. With the emergence of human relapsing fever cases in western Montana, we have the opportunity to explore the role of local vertebrate hosts at maintaining the spirochete in nature. By establishing additional isolates of the spirochete, we can also determine the genetic diversity and host associations at specific locations. For example, on Wild Horse Island we found two diverse genomic groups of B. hermsii infecting two individuals that slept in the same bed (Schwan et al., 2007;Emerg. Infect. Dis). Thus, additional work in the field will complement laboratory investigations examining the possible maintenance of diverse genetic types of spirochetes in vertebrates and ticks.